Disclaimer: The first part of this story's canon timeline has been rewritten to fit with the more aligned Disney Canon timeline. None of this is officially canon by both Disney and Lucasfilm.
Old Republic Era (circa BBY(Before Battle of Yavin))
25,000 BBY - The Formation of the Old Republic and Jedi Order
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7000 BBY - Dark Jedi rebel against the Jedi Order, sparking the Hundred-Year Darkness
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6900 BBY - Dark Jedi defeated and later exiled deep in the Outer Rim, eventually landing on Korriban; End of the Hundred-Year Darkness
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5000 BBY - The Great Hyperspace War begins and ends, thus destroying the Old Sith Empire; Sith Empire ruler Naga Sadow permanently went into exile on Yavin 4, never to be seen again
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3996 BBY - Exar Kun turns to the dark side and forms the Brotherhood of the Sith and his own Sith Empire; The Great Sith War (aka the Exar Kun War) begins and ends, death of Exar Kun and imprisonment of his spirit on Yavin 4; collapse of the Brotherhood of the Sith and Exar Kun's Sith Empire
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3976 BBY - Mandalore the Ultimate and his Neo-Crusaders begin raiding Althir III, sparking the Mandalorian Wars
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3963 BBY - An unnamed, charismatic young Jedi that'll later be renamed Revan leads a splinter group of Jedi rebels against the Jedi Council to take part in helping the Old Republic against the Mandalorians during the war, thus naming the Jedi movement the Revanchists; tide of the war slowly turns around
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3960 BBY - Mandalore the Ultimate single-handedly slayed by Revan above the moon Malachor V; the Mass Shadow Generator wipes out most of the Mandalorians on Malachor V, sacrificing hundreds of Jedi and Old Republic forces in the process and ending the Mandalorian Wars
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3659 BBY - A year after turning to the dark side on a hunt for the reformed Sith Empire in the Unknown Regions, Darth Revan and his apprentice Darth Malak located the Star Forge, and created their own Sith Empire; Revan's Sith Empire invades the Old Republic, beginning the Jedi Civil War
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3957 BBY - Revan is betrayed by Malak, leaving the former in a coma and the latter usurps the title of Dark Lord of the Sith; the Jedi Knight known as Bastila Shan brings the comatose Revan back to the Jedi Council and wiped his memories, creating a new identity for him and would later be retrained as a Jedi
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3956 BBY - The Battle of Rakata Prime occurs and the Star Forge that Revan's former Sith Empire used to start the Jedi Civil War is found; Revan redeems himself and becomes a Jedi once more and kills Malak; Star Forge is then destroyed and the Jedi Civil War ends; Sometime after Malak's death, Revan's former Sith Empire enters a power struggle, thus beginning the Sith Civil War on Korriban
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3954 BBY - Darth Traya (Revan's former Jedi Master-turned-Sith Lord) rose to power along with her two Sith apprentices known as Darth Nihilus (the Lord of Hunger) and Darth Sion (the Lord of Pain) and formed the Sith Triumvirate from the remnants of Revan's former empire on Malachor V; the First Jedi Purge and campaign against the Old Republic begins; Traya overthrown by Nihilus and Sion not long after the beginning of the purge
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3951 BBY - Nihilus killed on Citadel Station over Telos IV; Sion and Traya killed on Malachor V before entire moon itself is destroyed, ending the Dark Wars that began four years prior (3955 BBY); First Jedi Purge ends
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3950 BBY - Remnants of Revan's former Sith Empire finally collapsed after six years of constant infighting; End of the Sith Civil War
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3681 BBY - Thirteen centuries after the Great Hyperspace War, the newly rebuilt Sith Empire returned from the Unknown Regions to enact revenge against the Jedi Order and the Old Republic, taking Korriban back in the process; the Great Galactic War (aka the First Galactic War) begins
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3667 BBY - The Battle of Alderaan begins and ends with the defeat of Darth Malgus and his invasion army
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3653 BBY - The Sith Empire's Dark Council offers the Old Republic a chance to negotiate a peace treaty known as the Treaty of Coruscant, to which the Republic Senate and the Jedi couldn't ignore, so the peace talks took place on Alderaan; at the same time, however, the Sacking of Coruscant took place as a final card for the Sith Empire to play; the peace treaty is signed and the Great Galactic War comes to an end; The Cold War (Old Republic era) begins
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3642 BBY - Treaty of Coruscant broken and the conflict between the Sith Empire and the Old Republic begins again; the Galactic War (aka the Second Galactic War) begins
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3636 BBY - War comes at a stalemate when the Eternal Empire conquers the known galaxy that began a year prior (3637 BBY) with the near eradication of both the Sith Empire and Old Republic, forcing them to sign treaties with the Eternal Empire; the Eternal Empire becomes the dominant galactic power; the Galactic War ends
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3632 BBY - Revolt against the Eternal Empire begins
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3630 BBY - The former Sith Emperor, and later Eternal Emperor known as Vitiate (under the name Valkorian), dies for the last time on Zakuul in a final confrontation with the former Hero of Tython known as the Outlander; the Eternal Empire is reorganized into the Eternal Alliance; Sometime later, the Sith Empire attacks a hidden Jedi Colony on the Jedi library world of Ossus with Darth Malgus revealing to be alive after his supposed death years prior, thus sparking the Third Galactic War between the Sith Empire, the Old Republic, and the Eternal Alliance
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Prior to 3522 BBY - The Third Galactic War ends; collapse of the Eternal Alliance
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Prior to 1019 BBY - A battle takes place between the Jedi Order and the Sith Empire on the rocky wasteland planet of Malachor (not to be confused with the extinct moon of Malachor V), ending at a stalemate and is dubbed as the Great Scourge of Malachor; centuries later, the Jedi-Sith War begins when the Sith Empire took over Coruscant and the Old Republic falls; the Mandalorain-Jedi War begins
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1019 BBY - The Mandalorian-Jedi War ends with the planet Mandalore becoming a lifeless, white desert wasteland in the process; the Jedi-Sith War ends with the near eradication of the Sith, leaving Darth Bane as the sole survivor that would later go on to establish the Rule of Two; the Sith operate in secrecy for the next thousand years; the Jedi Temple on the planet is rebuilt years after the Sacking of Coruscant; Coruscant liberated after the Sith were defeated; the Old Republic is reorganized into the Galactic Republic, thus disbanding its military branches